1. Field of the Invention
The present invention relates to a reverse current preventing circuit and, more particularly, to a reverse current preventing circuit, which is applied in a synchronous switching voltage converter, for accurately preventing occurrence of current reversal.
2. Description of the Related Art
FIG. 1(a) is a circuit diagram showing a conventional synchronous switching voltage converter 10. The synchronous switching voltage converter 10 converts an input voltage Vin into a regulated output voltage Vout for being supplied to a load Ld. A high-side switch SH is coupled between the input voltage Vin and a switch node SN, while a low-side switch SL is coupled between the switch node SN and a ground potential. In the example shown in FIG. 1(a), the high-side switch SH is implemented by a PMOS transistor while the low-side switch SL is implemented by an NMOS transistor. An inductor L is coupled between the switch node SN and an output terminal O. A switch control circuit 12 generates a switch control signal CS, such as a pulse-width-modulation signal, for determining whether to turn ON/OFF the high-side switch SH and the low-side switch SL. In response to the feedback of the output voltage Vout, the switch control circuit 12 adjusts the duty cycle of the switch control signal CS so as to regulate the output voltage Vout. Furthermore, an output capacitor Co is coupled to the output terminal O so as to filter ripples of the output voltage Vout.
FIG. 1(b) is a waveform timing diagram showing an ideal operation of a conventional synchronous switching voltage converter 10. During time ta to tb, the high-side switch SH is turned ON and the low-side switch SL is turned OFF, such that an inductor current IL flows from the input voltage Vin through the high-side switch SH to the output terminal O and gradually increases. In this case, a switch voltage VSN at the switch node SN is pulled up to the input voltage Vin. At time tb, the high-side switch SH is turned OFF and the low-side switch SL is turned ON, such that the inductor current IL flows from the ground potential through the low-side switch SL to the output terminal O and gradually decreases. In this case, the switch voltage VSN at the switch node SN is abruptly pulled down below the ground potential. When the inductor current IL decreases to reach a predetermined threshold current Ith, i.e. at time tc, a potential difference across a sensing resistor RS caused by the inductor current IL becomes smaller than a reference voltage source Vref. As a result, the potential received at a non-inverting input terminal of a comparator CP1 is larger than the potential received at an inverting input terminal thereof. Because the comparator CP1 has a response time tR due to a finite speed of practical operation, it is necessary to wait until time td for a prevention signal PR1 to be changed from the low level L to the high level H at the output of the comparator CP1, wherein (td−tc)=tR. In response to the high level H of the prevention signal PR1 output from the comparator CP1, a blocking circuit 15 outputs a low level signal to turn OFF the low-side switch SL. Ideally, the moment when the blocking circuit 15 turns OFF the low-side switch SL is arranged at the event that the inductor current IL just reduces to zero, i.e. at time td, so as to prevent the inductor current IL from reversing the direction. In order to achieve such an ideal effect, the decreasing rate of the inductor current IL and the response time tR of the comparator CP1 should be precisely predicted so as to determine an appropriate threshold current Ith.
However, the decreasing rate of the inductor current IL is proportional to the output voltage Vout and is inversely proportional to the inductor L. It is assumed that the threshold current Ith is kept constant after manufacturing. If the decreasing rate of the inductor current IL is relatively too slow, the inductor current IL has not yet reduced to zero at the moment when the low-side switch SL is turned OFF, as shown in FIG. 2(a). If the decreasing rate of the inductor current IL is relatively too fast, the inductor current IL has already reversed the direction at the moment when the low-side switch SL is turned OFF, as shown in FIG. 2(b). In these two cases, the operational efficiency of the synchronous switching voltage converter 10 is subjected to deterioration.
Moreover, the threshold current Ith is, as a matter of fact, subjected to variation along with processing parameter shift and operational temperature change. If the threshold current Ith moves up to become a larger threshold current Ith′, as shown in FIG. 3(a), the low-side switch SL has already been turned OFF before the inductor current IL reduces to zero. If the threshold current Ith moves down to become a smaller threshold current Ith″, as shown in FIG. 3(b), the low-side switch SL has not yet been turned OFF at the moment when the inductor current IL reverses the direction. In these two cases, the operational efficiency of the synchronous switching voltage converter 10 is subjected to deterioration.